Vibration in built-up structures causes problems ranging from radiation of acoustic noise to damage to sensitive instrumentation. Built-up structures are any composite of beams, plates, joints and other structural components connected to form a nominal single unit. Examples of built-up structures are bridges, electronics cabinets, sports equipment, and automobiles. All of these structures suffer from vibrations to one degree or another. Structural vibration is caused most commonly by external forcing from impact, attached equipment, or fluid flow. In many structural applications it is desirable to reduce the level of such vibrations.
The use of granular material to effect structural damping is well known. Sand and lead shot have been used for some time to provide such damping. For example, sand is used to fill the hollow spaces of tubular beams. Sand and lead shot, though effective in providing damping primarily through a mass loading effect, are very heavy, so that their use extracts a high weight penalty. For example, sand has a bulk specific gravity of not less than 1.5 and lead shot has a bulk specific gravity of approximately 7.
The present inventor has previously established that some lighter weight materials can provide significant vibration damping. U.S. patent application Ser. No. 08/662,167 (allowed) discloses and claims the use of granular materials with a bulk specific gravity of less than 1.5, such as glass micro-spheres or polyethylene beads, for vibration damping. The teachings of this allowed application are incorporated herein by reference. The materials described therein do not rely upon the mass-loading effect of high-density materials, nor on the high intrinsic attenuation of viscoelastic materials. Rather, the dominant damping mechanisms are disclosed to be the small intrinsic attenuation within the solid pieces of granular material, the friction between pieces of granular material, and the non-linear hysteresis in the pieces of granular material arising from deformation during the wave propagation process. The effectiveness of these materials for vibration damping is attributed to a low bulk wave speed characteristic of granular materials.
Thus the prior art acknowledges several desirable characteristics for a granular vibration damping material. The material may have significant mass relative to the host structure such as sand or lead shot, high intrinsic damping characteristics such as viscoelastic spheres, or a low bulk wave speed which allows several other damping mechanisms to come into play. While studies have tried to increase each of material density and viscoelasticity to enhance damping, to date, little, if any, attention has been paid to the reduction of bulk wave speed of materials to improve their vibration damping characteristics.